Carbon product rich in lithium for use as negative electrode in a lithium cell

ABSTRACT

The invention relates to a lithium-rich carbonaceous substance which can be used as a negative electrode in a lithium accumulator. 
     This substance has the following formula: 
     
       
         LiNa x C y O z   (I) 
       
     
     in which x, y and z are such that 
     0.4≦x≦0.6 
     2.5≦y≦3.5 
     0.2≦z≦1 
     It can be prepared by the insertion of lithium electrochemically in a graphite-sodium-oxygen compound.

TECHNICAL FIELD

The object of the present invention is a lithium-rich carbonaceoussubstance comprising lithium, sodium, carbon and oxygen, which can beused in particular as a negative electrode in a lithium electricalaccumulator.

It applies to the production of accumulators with a high mass energywhich are of great interest for the development of portable electronicsand in the long term for the manufacture of electric vehicles.Currently, for these applications, the choice seems to relate to lithiumion batteries because of their very high potential energy per unit mass.For the anode part of these batteries, numerous studies have shown thatit is possible to use carbonaceous compounds and in particular graphite.

This is because graphite with a lamellar structure has the property,under the effect of an electric current in an appropriate electrolyticmedium, of inserting lithium ions in its structure. The substanceobtained is a graphite insertion compound, that is to say there is aninvasion of the vacant spaces between the graphite lamellae, alsoreferred to as van der Waals spaces, by the lithium ions.

At the present time, the insertion compound of lithium in graphite whichis the richest known has one lithium atom for six carbon atoms, whichcorresponds to the total formula LiC₆ and has an electrochemicalcapacitance of 372 mA.h.g⁻¹. This compound LiC₆ is described in Carbon,volume 13, pages 337-345, 1975 [1].

Through FR-A-2 697 261 [2], a polyacetylene-lithium insertion compoundis known in which the C/Li ratio is 6, as in the case of the insertioncompound of lithium in graphite LiC₆.

In order to improve the performance of lithium batteries, it wouldhowever be highly advantageous to use, as a negative electrode,compounds containing even more lithium than these known compounds, inorder to obtain a higher electrochemical capacitance.

DESCRIPTION OF THE INVENTION

The object of the present invention is precisely a novel carbonaceoussubstance containing more lithium than any known compounds, which can beused as a negative electrode in a lithium ion battery.

According to the invention, this lithium-rich carbonaceous substance hasthe following formula:

LiNa_(x)C_(y)O_(z)  (I)

in which x, y and z are such that:

0.4≦x≦0.6

2.5≦y≦3.5

0.2≦z≦1

The carbonaceous substance of the invention thus contains more lithiumthan the insertion compound of lithium in graphite LiC₆ since the C/Liratio is situated within the range 2.5 to 3.5, which corresponds to asignificant increase in lithium content and an increased electrochemicalcapacitance (˜744 mA.h per gram of carbon).

This lithium-rich carbonaceous substance can be obtained easily from agraphite-sodium-oxygen compound such as the one described by M. El Gadiet al in J. Mol. Cryst. Liquid. Cryst., 244-245, 1994, page 41[3].

The formula of this compound deduced from elemental analyses is:

C_(4.75±0.05)NaO_(0.35±0.05)

According to the invention, the lithium-rich carbonaceous compound isprepared by inserting lithium into a graphite-sodium-oxygen compound ofthis type. This insertion can be carried out either chemically orelectrochemically.

The insertion by chemical method can be effected by means of a methodsimilar to the one described by A. Essaddek et al in C. R. Acad. Sci.,Paris, 1. 319, Series II, 1994, pages 1009-1012 [4]), starting from thecompound NAO_(0.5)C₆ with an ideal theoretical formula derived fromcrystallographic analysis. In this case, the graphite-sodium-oxygencompound is put in contact with liquid lithium at a temperature and fora period sufficient to obtain the lithium-rich carbonaceous substance ofthe invention.

It is also possible to start from a graphite-sodium-oxygen compoundprepared by putting in contact, in a sealed chamber containing nooxygen, graphite with sodium containing a small quantity of oxygen, at atemperature of 460 to 480° C., and preferably 470° C.

The graphite used for this preparation can be natural or artificialgraphite, in flake or powder form with variable granulometries.

By way of examples of graphites which can be used, pyrographite of thePGCCL type supplied by Carbone Lorraine and highly orientatedpyrographite HOPG supplied by Union Carbide can be cited.

The small quantity of oxygen present in the sodium can represent 0.5 to2% by weight for a volume of sodium of 1 to 2 cm³.

Preferably, according to the invention, the insertion of the lithium inthe graphite-sodium-oxygen compound is effected electrochemically. Inthis case, an electrolytic cell with two electrodes is used, one ofwhich is made of lithium and the other one of which consists of thegraphite-sodium-oxygen compound, the two electrodes being immersed in anelectrolyte comprising a lithium salt.

The electrolyte generally comprises a non-aqueous solvent, for exampleethylene carbonate. The lithium salts which can be used may be ofdifferent types, but lithium perchlorate is preferred.

In order to effect the electrochemical insertion, the operation ispreferably carried out at a temperature of 20 to 90° C. and a potentialof 0 volts is imposed between the two electrodes, which can be achievedby connecting the two electrodes by simple short-circuit.

The lithium-rich substance of the invention can be used in particular asa negative electrode in a lithium electrical accumulator.

Thus another object of the invention is an accumulator comprising anegative electrode based on lithium, a positive electrode and anelectrolyte conductive by lithium ions in which the negative electrodecomprises the lithium-rich carbonaceous substance of the invention.

In this accumulator, the positive electrode can be produced from variousmaterials such as oxides, sulphides or oxysulphides.

By way of example of oxides which can be used, it is possible to citevanadium oxide V₂O₅, nickel oxide NiO₂, cobalt oxide CoO₂, mixed oxidesof cobalt and nickel, manganese oxides, molybdenum oxide MoO₃, chromiumoxides and vanadium bronzes MxV₂O₅ with M representing iron, sodium,potassium, lithium, silver, aluminium, chromium, barium, nickel orcobalt.

By way of examples of sulphides which can be used, titanium sulphideTiS₂, molybdenum sulphide MoS₂ and mixed sulphides of nickel andmolybdenum can be cited.

By way of examples of oxysulphides which can be used, molybdenum andtitanium oxysulphides can be cited.

In this electrical accumulator, the electrolyte used generally consistsof a solution of lithium salt in a suitable organic solvent.

The organic solvents which can be used are for example propylenecarbonate, ethylene carbonate, dimethyl carbonate (DMC), methyl ethylcarbonate (MEC), tetrahydrofuran, 2-methyltetrahydrofuran,dimethoxymethane, dimethoxyethane, N,N-dimethylformamide, sulfolane andmixtures thereof.

The lithium salts which can be used are for example lithium perchlorateLiClO₄, lithium hexafluorophosphate LiPF₆, lithium hexafluoroarseniateLiAsF₆, lithium trifluoromethanesulfonate LiCF₃SO₃ and lithiumtetrafluoroborate LiBF₄.

In the electrolytic accumulator, it is also possible to use as anelectrolyte, instead of a solution of lithium salt in an organicsolvent, solid electrolytes or polymers conductive by lithium ions.

By way of examples of solid electrolytes, lithium glasses can be cited,obtained for example from P₂S₅, Li₂S and LiI or B₂S₃, Li₂S and LiI.

The polymers conductive by lithium ions can consist for example ofpoly(ethylene oxide) or poly(propylene oxide) containing a lithium saltsuch as the salts described above.

In an accumulator of this type using a liquid electrolyte, generally aseparator is disposed between the electrodes, and this can consist of amicroporous film produced for example from polypropylene orpolyethylene.

This accumulator can be produced in the form of a cylindricalaccumulator having a spiral winding of the two electrodes separatedpossibly by the separator. It can also be produced in the form of anaccumulator of the prismatic type with plane electrodes facing eachother and possibly a separator disposed between these electrodes.

Other characteristics and advantages of the invention will emerge moreclearly from a reading of the following description, given of course forillustrative purposes and non-limitatively, with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the X-ray diffraction spectrum giving the 001 reflectionsof the graphite-sodium-oxygen compound prepared in Example 1;

FIG. 2 depicts the X-ray diffraction spectrum presenting the hk0reflections of the graphite-sodium-oxygen compound of Example 1;

FIG. 3 depicts an X-ray diffraction spectrum presenting the 001reflection of the lithium-rich substance of the invention;

FIG. 4 depicts the X-ray diffraction spectrum presenting the hk0reflections of the lithium-rich substance of the invention;

FIG. 5 illustrates the X-ray emission spectrum (EDX) of the lithium-richsubstance of the invention;

FIGS. 6 to 8 illustrate the electron energy loss spectra (EELS) of thelithium-rich substance of the invention and confirm the presence ofoxygen (FIG. 6), sodium (FIG. 7) and lithium (FIG. 8);

FIG. 9 is a curve illustrating the differential thermal analysis of thelithium-rich compound of the invention;

FIG. 10 is a schematic representation of an electrical accumulatoraccording to the invention.

DETAILED DISCLOSURE OF EMBODIMENTS EXAMPLE 1 Synthesis of theGraphite-sodium-oxygen Compound

In order to prepare this compound use is made of a steel reactor havingtwo compartments placed one above the other and separated by a metallicgrille. In the bottom compartment, metallic sodium containing a smallquantity of oxygen is introduced, and in the top compartmentpyrographite wafers of the PGCCL type supplied by Le Carbone Lorraineare deposited. This reactor was assembled in a pure argon atmosphere,that is to say one free of any trace of oxygen. The airtightness of thereactor was ensured by means of an O-ring seal made of copper, and, inorder to prevent accidental contamination of the reaction medium, thereactor is placed in a glass tube in which a primary vacuum is produced.The assembly consisting of the tube containing the reactor is thenplaced in a vertical oven and is raised to a temperature of 200° C. for12 hours. The sodium, with slight oxygen content, is then in the liquidstate.

The reactor is then removed from its tube and is placed immediately in acentrifuge. After centrifugation for 15 minutes at 5000 rev/min, theliquid reaction medium invades the compartment containing the sample ofpyrographite. The reactor is then replaced in the reverse position, thatis to say by placing the top compartment at the bottom, and it is putback in the tube, in which a primary vacuum is produced. The reactionmixture is then heated at a temperature of 470° C. for three days, andthen the graphite is separated from the reaction medium bycentrifugation under the same conditions as before. In this way ablue-coloured graphite-sodium-oxygen compound is obtained.

This compound is characterised by X-ray diffraction using a conventionalθ/20 diffractometer having a molybdenum source (·=0.72926 pm) and aquartz monochromator, mounted in series, and finally a scintillationcounter. The sample is placed in a pure argon atmosphere in a Lindemantube, which is then sealed.

FIG. 1 illustrates the X-ray diffraction spectrum presenting the 001reflections of this compound. This diffractogram makes it possible tocalculate the identity period of the compound, which is 1080 pm.

FIG. 2 depicts the spectrum corresponding to the hk0 reflections of thiscompound. It shows the 100 and 110 peaks of graphite as well as thepeaks of lower intensity due to the insertion of sodium accompanied byoxygen between the graphite flakes (ternary compound).

An electron diffraction analysis of this compound is carried out on asample of crushed pyrographite in a pure argon atmosphere which isplaced on an amorphous carbon grille with holes. This analysis revealsspots due to the graphite as well as spots due to the sodium insertedbetween the graphite planes.

EXAMPLE 2 Preparation of a Lithium-rich Substance of FormulaLiNa_(x)C_(y)O_(z) (with x=0.4-0.6, y=2.5-3.5 and z=0.2 to 1)

This substance is obtained by electrochemical insertion of lithium whosegraphite-sodium-oxygen compound was prepared in Example 1.

For this purpose, a sealed cell with two electrodes is used, in which aprimary vacuum is produced.

One of these two electrodes consists of a lithium ribbon held in atitanium clamp serving as a reference and counter-electrode, the otherelectrode, or working electrode, consists of the blue colouredgraphite-sodium-oxygen compound prepared in Example 1, which is alsoheld by a titanium clamp.

The two electrodes are immersed in an electrolytic mixture consisting ofethylene carbonate (EC) and lithium perchlorate LiClO₄ in the followingproportions:

1.5 mol of LiClO₄ per kg of ethylene carbonate.

The two constituents of the electrolyte are degassed under vacuum beforebeing mixed. The lithium salt LiClO₄ was degassed under primary vacuumat 150° C. for 4 hours; the ethylene carbonate solvent was degassedunder primary vacuum at room temperature for two hours.

In order to effect the electrochemical insertion of the lithium ions,the operation is carried out at a temperature of 20 to 90° C., forexample at 80° C., imposing a potential between the two electrodes of 0volts by means of a potentiostat/galvanostat. It is also possible toeffect the insertion by simple short-circuit, and therefore byconnecting the two electrodes.

The progress of the reaction is governed by kinetic problems: it dependson the specific surface area of the graphite for a given concentrationof the electrolyte and also the temperature.

After a maximum reaction time of 30 days, the lithium-rich substance isobtained, whose theoretical composition formula is as follows:

LiC₃Na_(0.5)O_(0.25)

This substance is characterised by X-ray diffraction and by electrondiffraction as in Example 1.

FIG. 3 depicts the diffractogram for the 001 reflections of thislithium-rich substance. From this diffractogram the identity period ofthe substance is calculated, which is around 1035 pm. In addition to the001 reflections, the diffractogram shows the 110 reflection of themetallic sodium.

FIG. 4 depicts the diffractogram for the hk0 reflections of thissubstance. This exhibits the characteristic 110 and 100 reflections ofgraphite as well as 211 and 110 reflections of metallic sodium.

The electron diffraction analysis shows visible spots due to thegraphite as well as fine spots attributed to the metallic sodium. Inaddition a new spot is found due to the insertion of the lithium.

FIG. 5 depicts the X-ray emission spectrum of this substance. It showsthe following elements: carbon, oxygen and sodium.

FIGS. 6 to 8 depict the energy loss spectra of the electrons whichconfirm the presence of carbon and oxygen (FIG. 6), sodium (FIG. 7) andlithium (FIG. 8).

Quantitative analysis of these elements detected by this techniqueconfirms the elemental analysis carried out initially, that is to saythe formula

LiNa_(x)C_(y)O_(z)

with:

x=0.4-0.6

y=2.5-3.5

z=0.2-1

corresponding to the ideal formula: LiC₃Na_(0.5)O_(0.25).

FIG. 9 depicts the curve for differential thermal analysis in an argonatmosphere of the substance obtained. On this curve there can be seenthe fusion peak corresponding to the fusion of the metallic sodium,which confirms the presence of sodium in metallic form in this compound.

EXAMPLE 3 Preparation of a Lithium-rich Substance of FormulaLiNa_(x)C_(y)O_(z) (x=0.4-0.6, y=2.5-3.5, z=0.2-1)

In this example, the starting point is the graphite-sodium-oxygencompound of Example 1, and lithium is inserted in it chemically.

For this purpose, a two-compartment reactor of the same type as inExample 1 is used and the blue coloured graphite-sodium-oxygen compoundis placed in the top compartment and the bottom compartment is filledwith lithium.

The operation is carried out as in Example 1 by placing the reactor in aglass tube in which a primary vacuum is produced, and then the liquidlithium and the compound are brought into contact by centrifugation at5000 rev/min for fifteen minutes. To melt the lithium, the whole isheated at 200° C. for twelve hours, and the reaction is carried out at200° C. for a maximum period of ten days.

In this way the lithium-rich substance of formula LiNa_(x)C_(y)O_(z) isobtained, which has the same characteristics as that obtained in Example2.

Thus the invention makes it possible to obtain a substance with a muchgreater lithium content than LiC₆, which results in a doubling of theelectrochemical capacitance of the graphite. In this substance, thelithium inserted in the graphite-sodium-oxygen compound is thereforesubstituted for the sodium, which then forms metallic inclusions betweenthe graphite wafers.

In this substance there are successive layers interposed between thegraphite wafers which do not have the same chemical composition, that isto say Li/O/Li/O/Li layers where O is the oxygen engaged in a peroxideion O₂ ²⁻, and layers of Li, with one layer of Li for five layers ofLi/O/Li/O/Li. The sodium is situated in inclusions diffused in thesubstance.

Such a substance can be used in a lithium accumulator as a negativeelectrode.

FIG. 10 depicts a lithium electrical accumulator using the lithium-richcarbonaceous substance of the invention, with the formula:

LiNa_(x)C_(y)O_(z)  (I).

In this figure, it can be seen that the accumulator comprises areceptacle 1 made of polytetrafluoroethylene, filled with an electrolyte3 consisting for example of ethylene carbonate containing 1 mol/l oflithium perchlorate, in which there are disposed successively a negativeelectrode 5 consisting of the lithium-rich carbonaceous substance offormula (I) of the invention, a separator 7 consisting of a microporouspolypropylene membrane, and a positive electrode 9 made of vanadiumoxide.

REFERENCES CITED

[1] Carbon, Vol. 13, 1975, pages 337-345

[2] FR-A-2 697 261

[3] J. Mol. Cryst. Liquid Cryst., 244-245, 1994, page 41

[4] C. R. Acad. Sci. Paris. t. 319, Series II, 1994, pages 1009-1012

What is claimed is:
 1. A lithium-rich carbonaceous substance with diefollowing formula: LiNa_(x)C_(y)O_(z)  (I) in which x, y and z are suchthat 0.4≦x≦0.6 2.5≦y≦3.5 0.2≦z≦1.
 2. A lithium electrical accumulatorcomprising a negative electrode based on lithium, a positive electrodeand an electrolyte which is conductive by lithium ions, in which thenegative electrode comprises a lithium-rich carbonaceous substance withthe formula: LiNa_(x)C_(y)O_(z)  (I) in which x, y and z arc such that0.4≦x≦0.6 2.5≦y≦3.5 0.2≦z≦1.
 3. A method of preparing a lithium-richcarbonaceous substance with the following formula:LiNa_(x)C_(y)O_(z)  (I) in which x, y and z are such that 0.4≦x≦0.62.5≦y≦3.5 0.2≦z≦1 according to which a graphite-sodium-oxygen compoundis put in contact with lithium in order to insert lithium ions therein.4. A method according to claim 3, in which the insertion is carried outelectrochemically in an electrolytic cell with two electrodes, one ofwhich is made of lithium and the other from the graphite-sodium-oxygencompound, the two electrodes being immersed in an electrolyte comprisinga lithium salt.
 5. A method according to claim 4, in which theelectrolyte is a mixture of ethylene carbonate and lithium perchlorate.6. A method according to claim 4, in which the operation is carried outat a temperature of 20 to 90° C., by imposing a potential of 0 voltbetween the two electrodes or connecting the two electrodes.
 7. A methodaccording to claim 3, in which the graphite-sodium-oxygen compound hasthe formula C_(4.75)±_(0.05)NaO_(0.35)±_(0.05).
 8. A method according toclaim 3, in which the graphite-sodium-oxygen compound is prepared bypuffing in contact, in a scaled chamber containing no oxygen, graphitewith liquid sodium containing a small quantity of oxygen, at atemperature of 460 to 480° C.